Good advice comes with a rationale so you can tell when it becomes bad advice. If you don't understanding why something should be done, then you've fallen into the trap of cargo cult programming, and you'll keep doing it even when it's no longer necessary or even becomes deleterious.

The person who wrote this is a very smart fellow which usually writes well, so this makes me wonder if it's an idiom I don't know instead of a simple (and ugly) mistake.

@vonjd: Sure, and I was almost certain it was one in this case, but it has happened before that things I thought were mistakes, weren't after all. So, never hurts to ask.
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Vinko VrsalovicOct 27 '10 at 8:44

As a native speaker, I had to re-read this question several times to see what was being asked, because my brain deleted the -ing on "understanding" even though it's in the headline AND the quotation.
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Mr. Shiny and New 安宇Oct 27 '10 at 17:27

Thought so... It could come from having written at first "If you aren't understanding why..."
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Vinko VrsalovicOct 27 '10 at 5:32

I think you'd probably say 'If you don't understand why' rather than if you aren't understanding why'. The latter usage is (in the UK anyway) a textbook example of a mistake someone (often those with a South Asian language as their Mother Tongue) might make whilst speaking English.
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5arxJul 24 '12 at 10:30